1. Field of the Invention
The present invention relates to an actuator having a piezoelectric drive mechanism using a piezoelectric thin film. In particular, the present invention relates to a micro-electromechanical system device using a piezoelectric actuator.
2. Description of the Related Art
Recently, radio-frequency (RF) micro-electromechanical system (MEMS) devices such as a variable capacitor and a micro switch, which use actuators fabricated by MEMS technology, are being studied. The variable capacitor or the micro switch fabricated by the MEMS technology includes a movable electrode provided on a beam of an actuator having a movable end suported on a substrate, and a fixed electrode provided on a surface of the substrate, which faces the actuator. In the actuator, the beam is bent and displaced by a drive force, such as an electrostatic force, a thermal stress, an electromagnetic force, and a piezoelectric force, to vary the distance between the movable electrode and the fixed electrode. A variable capacitor and a micro switch using a piezoelectric actuator have been proposed (refer to Japanese Unexamined Patent Publication No. 2004-6588 and U.S. Pat. No. 6,359,374). The piezoelectric actuator using a piezoelectric thin film has the advantages of a low drive voltage and low power consumption. Accordingly, RF-MEMS devices having a piezoelectric actuator are suitable for a mobile instrument, such as a mobile phone.
A beam of the piezoelectric actuator has a multilayer structure including a piezoelectric film sandwiched by a top electrode and a bottom electrode. When the piezoelectric film is contracted or expanded by applying a voltage between the top and bottom electrodes, the beam is bent. Using the displacement obtained by bending the beam, the piezoelectric actuator may be operated as a variable capacitor or a micro switch.
When the piezoelectric actuator is used, it is necessary to prevent the beam from warping due to factors other than the intended expansion and contraction of the piezoelectric film. However, in the multilayer beam structure, warpage occurs due to various factors. For example, since individual films of the multilayer structure have different thermal expansion coefficients, the beam warps due to temperature changes. Further, when each film of the multilayer structure is deposited, different stresses occur in each film depending on the deposition method and material of the film. When the summation of the moments of membrane stress in the individual films is not zero, the beam warps.
The warpage of the beam due to the differences in thermal expansion coefficients can be prevented by stacking the films in such a manner that the distribution of the thermal expansion coefficients of the respective films is vertically symmetrically arranged in the stacking direction. However, in a unimorph-type piezoelectric actuator, material and thickness of each film is vertically asymmetrical in the stacking direction in order to bend the beam by a piezoelectric effect. Accordingly, it is difficult to control the distribution of the thermal expansion coefficients of the films to be vertically symmetric.
On the other hand, warpage due to a residual stress can be reduced by adjusting the multilayer structure and the thicknesses of the respective films so that the summation of the moments of stress in the multilayer structure is a predetermined value or less. Further, it is also necessary to take into consideration thickness variations in deposition of the films in the multilayer structure.
In the case of a typical piezoelectric actuator used in an RF-MEMS device, a length of a beam is approximately 100 μm, and the amount of displacement due to a piezoelectric effect is several μm. In a piezoelectric actuator having such dimensions, the thickness variation needs to be limited within one percent in order to control the amount of warpage, due to the residual stress, within 0.1 μm. The required magnitude of thickness variation is less than the thickness variation generated in a general semiconductor manufacturing process. In order to achieve the required magnitude of thickness variation, a special deposition system and a special process control are necessary. Further, it is necessary to control not only the thickness variation but also other factors relating to warpage, e.g., magnitudes of Young's modulus and the membrane stress in each film. Thus, in the manufacture of a piezoelectric actuator, many factors need to be controlled, and the cost of the end product increases.